widgetDragDestFindTarget introspection data does not show nullable result.
The documentation for widgetDragDestFindTarget says:
Looks for a match between the supported targets of context and the destTargetList, returning the first matching target, otherwise returning GDK_NONE.
However the introspection data does not include the possible null result.
This was originally found in the Haskell binding here: https://github.com/haskell-gi/haskell-gi/issues/138